In a packet-based system, such as 3rd Generation Partnership Project-Long Term Evolution (3GPP-LTE) system, a signal transmitted to a terminal is transmitted by a specific time unit, such as a Transmission Time Interval (TTI). Therefore, a signal transmitted to one terminal may or may not be present in every TTI. Information as to whether data transmitted to a terminal in each TTI is present or not is transferred through a control signal transmitted in an early part of the TTI. The terminal determines whether data transmitted within the TTI is present by decoding the control signal, and acquires information used for decoding the data. The terminal decodes a data signal in a latter part of the TTI excluding the control signal. Therefore, the terminal is unable to determine in advance whether a signal is transmitted to the terminal within each TTI, and thus, the terminal should decode the control signal in every TTI. Resultantly, even though data is not actually transmitted, the terminal consumes power by operating hardware for receiving a control signal in every TTI.
A wireless communication system including 3GPP supports a Discontinuous Reception (DRX) mode in order to reduce power consumption of a terminal. An LTE system supports the DRX mode not only in a Radio Resource Control (RRC)_IDLE state but also in an RRC_CONNECTED state to more actively control power. In this case, the terminal does not always monitor the control signal of TTI but discontinuously monitor the control signal of TTI during a determined interval, through the DRX operation. Here, the control signal may be referred to as a “Physical Downlink Control CHannel (PDCCH)”.
FIG. 1 illustrates a DRX cycle in a wireless communication system according to the related art.
Referring to FIG. 1, a DRX cycle 100 includes a reception interval 110, and a non-reception interval 120. Here, the reception interval 110 and the non-reception interval 120 may be referred to as an on-duration and an opportunity for DRX, respectively. The reception interval 110 is a downlink subframe time during which a terminal is awake and on standby in order to receive PDCCH in a DRX mode. During the reception interval 110, if there is no successfully decoded PDCCH, the terminal enters a sleep mode until a next reception interval 110 starts. On the other hand, if the PDCCH is successfully decoded, the terminal operates an inactivity timer according to conditions, and is awake until the inactivity timer expires.
The inactivity timer means a downlink subframe time, which is a waiting time from the last successful decoding of PDCCH to the newly successful decoding of PDCCH by the terminal. For example, during an on-state of the inactivity timer, the terminal attempts to decode the PDCCH while continuing to be awakened, and, if the PDCCH is not successfully decoded until the inactivity timer expires, the terminal enters a sleep mode. The terminal starts or restarts the inactivity timer after successful decoding of each PDCCH which is not re-transmitted but newly transmitted.
A whole time when the terminal is awake during the DRX mode is called an active time. The active time includes the reception interval 110 of the DRX cycle 100, a time during which the terminal continues to receive until the inactivity timer expires, and a time during which the terminal continues to receive while waiting for downlink re-transmission after one Hybrid Automatic Repeat reQuest (HARQ) Round Trip Time (RTT). Accordingly, the minimum of the active time is equal to the reception interval 110 of the DRX cycle 100, but the maximum of the active time is not limited.
When the DRX mode is set, the terminal operates according to a DRX procedure in every TTI. The TTI may be defined as one subframe having a length of 1 ms. Accordingly, there is a time interval during which a signal is not transmitted from a base station to the terminal according to mutually agreed patterns between the terminal and the base station, and thus, the terminal can save power by controlling power associated with reception during the time interval.
To effectively save power through the DRX mode, the DRX operation should be performed by a small number of TTIs, such as a short DRX. However, the short DRX cannot perfectly respond to data allocation properties varying actually and dynamically since the base station and the terminal are performed in a prescheduled manner. For example, due to the limitation in DRX scheduling and the mismatch between actual data allocation properties, the power saving efficiency of the DRX operation is slightly lowered.
Therefore, a need exists for an apparatus and a method for improving power saving performance of a terminal in a wireless communication system.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.